1. Field
An inrush current preventing circuit is provided.
2. Related Art
Generally, a switching power source apparatus comprises an inrush current preventing circuit to prevent large current (inrush current) generated instantaneously at the time of starting of an electronic equipment connected to the switching power source apparatus from flowing directly to the electronic equipment. In this configuration, an electronic equipment connected to the switching power source apparatus can avoid dangerousness that affects an electric system of the electronic equipment by unnecessarily operating a fuse or a circuit breaker of the electronic equipment.
As shown in FIG. 4, a conventional switching power source apparatus, as an example, is formed by a direct-current power source 110 that supplies direct current. An inrush current preventing circuit 101 prevents an inflow of inrush current. A DC-DC converter circuit 113 converts a voltage of the direct-current power source 110, so as to be connected in series to one another (referring to Patent Document 1).
An inrush current preventing circuit 101 has a switching device 102 that operates a switch by an applied voltage of MOS (Metal-Oxide-Semiconductor) type FET (Field Effect Transistor) or the like. The switching device 102 has an input terminal 102a that inputs current from a direct-current power source 110. An output terminal 102b outputs the inputted current to a DC-DC converter circuit 113. A control terminal 102c applies a voltage, which controls current between the input terminal 102a and the output terminal 102b. 
In the inrush current preventing circuit 101, a condenser 104 is connected in parallel between the input terminal 102a and the control terminal 102c of the switching device 102 and also a second resistor 103 is connected in parallel therebetween through a first resistor 105. The inrush current preventing circuit 101 has a discharge switching device 108 connected to a ground downstream of the first resistor disposed to the control terminal 102c. A voltage-controlled type switching device such as MOS type FET, similarly to the switching device 102, is used as the discharge switching device 108, and a discharge switch control means 109, which applies a voltage to the discharge switching device 108 in accordance with a predetermined setting, is connected to a control terminal 108c of the discharge switching device 108.
As described above, in a conventional inrush current preventing circuit 101 of a switching power source apparatus 100, since a condenser 104 is connected in parallel to a switching device 102, the inrush current preventing circuit 101 starts a direct-current power source 110 and can charge the condenser 104 connected in parallel to the switching device 102 when the discharge switching device 108 becomes a conduction state by an applied voltage from a discharge switch control means 109.
As shown in FIG. 5, when the condenser 104 charges, the voltage V (hereinafter, referred to as “a voltage of a switching device”), which is applied between the input terminal 102a and the control terminal 102c, gradually increases from an initial voltage V0 to a predetermined voltage V1. The increase of the voltage V depends on a time constant t obtained by the product of a resistance R of the first resistor 105 and a capacitance C of the condenser 104. Therefore, although a step-shaped voltage Vi is applied from a direct-current power source 110, the voltage V of the switching device 102 does not increase to the predetermined voltage V1 from the time t0 when a switching power source apparatus 100 starts until the predetermined time passed by a delay time td passes t1. Further, when the voltage V of the switching device 102 becomes the predetermined voltage V1 (when the predetermined time passes t1), the switching device 102 becomes a desired conduction state.
As shown in FIG. 5, since the conventional inrush current preventing circuit 101 of the switching power source apparatus 100 becomes a non-conduction state from the time t0 when the switching power source apparatus 100 starts until the predetermined time passed by a delay time td passes t1, the inrush current preventing circuit 101 prevents a inrush current, which is easily generated when the direct-current power source 110 starts, from inflowing to a load-resistor 112.
As shown in FIG. 4, in the conventional inrush current preventing circuit 101 of the switching power source apparatus 100, since a ground 107 is connected to an end of the discharge switching device 108, the inrush current preventing circuit 101 can completely discharge the condenser 104 from the time t2 when the direct-current power source 110 stops until the delay time td passes and completely discharged t3. In this configuration, although the direct-current power source 110 restarts after the discharged time t3, inrush current can be prevented.
The conventional inrush current preventing circuit 101 of the switching power source apparatus 100 above described needs a predetermined time (a delay time) td for discharging a condenser 104 completely. Therefore, there has been a problem that, when a direct-current power source 110 restarts immediately after the stop thereof, a voltage, which is larger than that V0 at the time of complete discharge of the condenser 104, is applied to a switching device 102.
As shown in FIG. 6, since a condenser is not discharged completely, a voltage V3 when the direct-current power source 110 restarts t4, from the time t2 when the direct-current power source 110 stops until the delay time td passes, becomes a value between a voltage V0 at the time of complete discharge t0 and a voltage V1 at the time of complete charge t2. The voltage V3 at the time of restart t4, as comes near the stop of the direct-current power source 110, becomes a value that is approximate to a voltage V1 at the time of complete charge t2. Therefore, when a direct current supplied from the direct-current power source 110 is a clock signal with a high duty ratio, a voltage V3 of the switching device 102 at the time of restart t4 increases.
A voltage-controlled type switching device 102 is configured to flow current that corresponds to an applied voltage without flowing current after a predetermined voltage V1 is applied. Therefore, when the voltage V3 is applied to the switching device 102 at the time of restart t4 of the direct-current power source 110, the direct-current power source 110 can not avoid current passing. As the voltage V3 is applied to the switching device 102 at the time of restart t4 of the direct-current power source 110 becomes a high value, a voltage passing the switching device 102 becomes inrush current which can affect a load-resistor 112 and then flows to the load-resistor 112.
In other words, in the conventional inrush current preventing circuit 101, since inrush current that corresponds to the voltage V3 at the time of restart t4 of the direct-current power source 110 flows to the switching device 102, the inrush current which passed the switching device 102 affects the load-resistor 112.